An organic EL device is a spontaneously luminous device which features higher brightness and higher legibility than those of the liquid crystal devices enabling vivid display to be attained and has, therefore, been vigorously studied.
In 1987, C. W. Tang et al. of the Eastman Kodak Co. have developed a device of a layer-laminated structure comprising various kinds of materials to bear individual roles, and have put an organic EL device using organic materials into a practical use. The above device is constituted by laminating layers of a phosphorescent body capable of transporting electrons and an organic material capable of transporting holes. The device is capable of attaining a brightness of as high as 1000 cd/m2 or more with a voltage of not higher than 10 V by injecting the above two kinds of electric charges into the layer of the phosphorescent body to emit light (see patent documents 1 and 2).
So far, many improvements have been made to put the organic EL device to practical use. For example, the organic EL device has been widely known having a structure comprising an anode, a hole injection layer, a hole-transporting layer, a luminous layer, an electron-transporting layer, an electron injection layer and a cathode which are arranged in this order on a substrate more finely dividing their roles than ever before. The device of this kind is achieving a high efficiency and a high durability.
To further improve the luminous efficiency, attempts have been made to utilize triplet excitons and study has been forwarded to utilize a phosphorescent luminous compound.
In the organic EL device, the electric charges injected from the two electrodes recombine together in the luminous layer to emit light. Here, to improve the luminous efficiency, to lower the driving voltage and to lengthen the life, it is necessary that the device has excellent carrier balance enabling the electrons and holes to be efficiently injected and transported, and enabling them to be efficiently recombined together.
As the hole injection material used for the organic EL device, there were, first, proposed phthalocyanines such as copper phthalocyanine (CuPc) (e.g., see a patent document 3), but materials having a phenylenediamine structure have now been widely used (see a patent document 4) because they have an absorption in the visible band.
As the hole-transporting material, on the other hand, arylamine materials having a benzidine skeleton have heretofore been used (see a patent document 5).
Tris(8-hydroxyquinoline) aluminum (Alq3) which is a representative luminous material has been generally used as the electron-transporting material. However, the electron mobility of the Alq3 is lower than that of the hole-transporting material that is generally used. Besides, the work function of the Alq3 is 5.8 eV which cannot be said to be a sufficiently large hole blocking power. Therefore, use of the above hole-transporting material is accompanied by a problem in that the holes partly pass through the luminous layer to deteriorate the efficiency.
In order to efficiently inject the holes or the electrons from the anode and cathode into the luminous layer, further, there has been developed a device obtained by laminating the hole injection layers and the electron injection layers each in a number of two or more layers to set stepwise the ionization potential values and the values of electron affinity possessed by the materials (see a patent document 6). With the materials that are used, however, none of the luminous efficiency, driving voltage or device life is still satisfactory.
Further, with the conventional organic EL devices, the hole-transporting layer usually consists of a very thin film. Therefore, the conventional organic EL devices are affected by the surface roughness of the transparent electrode such as ITO electrode which is used as the anode and a probability of producing defective products is high due to short-circuiting of the fabricated devices. In this case, an increase in the thickness of the hole-transporting layer can conceal the surface roughness of the anode such as ITO electrode and can decrease the probability of producing defective devices that are fabricated. However, the driving voltage increases with an increase in the thickness of the hole-transporting layer and may exceed a practical driving voltage. Namely, it becomes difficult to emit light with the practical driving voltage.
In order to improve characteristics of the organic EL device and to improve the yield of the device production, it has been desired to develop a device that features a high luminous efficiency, a low driving voltage and a long life by using in combination the materials that excel in hole and electron injection/transport property, stability and durability in the form of thin films, permitting holes and electrons to be highly efficiently recombined together.
Further, in order to improve characteristics of the organic EL device, it has been desired to develop a device that maintains carrier balance and features a high efficiency, a low driving voltage and a long life by using in combination the materials that excel in hole and electron injection/transport property, and stability and durability in the form of thin films.